This invention relates to a method and apparatus for improving efficiency of radio channel usage in overlapping coverage areas, for example, overlapping satellite beams.
With the trend toward increasingly small cells in cellular communications systems and the use of spot beams in satellite systems, areas of coverage overlap inevitably occur. In such areas, frequency re-use cannot occur if the systems are sensitive to interference. For example, consider a primary system of many users distributed over a service area, transmitting information back on a single channel to a common hub receive station. Assume there is also a secondary communications system wishing to utilize the same communications channel, again from many users to a common hub receive station, having a coverage area that at least partially overlaps the primary system. In order for the secondary system not to interfere with the primary system, conventionally under such circumstances the two systems would be allocated different channel frequencies. Given that in the case of satellite channels, the annual cost of 20 KHz bandwidth can be in excess one million dollars, this can be a very inefficient solution when the loading of the channels is not high.
The growth in demand for wireless communications systems has made spectrum space an extremely valuable commodity. For mobile satellite services (MSS), this demand has fueled proposals for numerous second and third generation mobile satellite systems. The increase in demand for spectrum that can support the various wireless services has resulted in a desire to more efficiently use existing spectrum. Clearly, the reuse of previously allocated spectrum without adversely affecting existing systems represents an increase in the utilization and efficiency of spectrum.
System design trade-offs for the mobile user have led to the deployment of satellite communications systems featuring low antenna gain terminals and, with the second and third generation of satellite systems, high gain satellite spot beams. As a consequence of the terminal mobility, the orientation between the terminal and the satellite can change significantly and over short periods of time.
A further consequence of using a low gain antenna at the terminal is that frequency reuse by other satellites and systems is severely limited. The transmissions from a low antenna gain terminal may be received by any satellite which covers the same frequency band and same coverage area as an existing satellite system.
Satellites and satellite systems which share common spectrum may not share the same geographical coverage when serving terminals with low gain antennas. Transmissions from users of a first satellite system, though transmitted using a low antenna gain, may not be received by a second satellite system operating at the same frequency band if there are differences in the satellite antenna ground coverage, and therefore may not interfere with users of the second satellite system. As a consequence of only partial overlap in geographical coverage between an existing satellite communications system and a second satellite communications system, a satellite communications channel on an existing satellite system that is heavily used, in the sense that the channel is occupied for a significant fraction of time, may appear to a second satellite system to be lightly loaded, in the sense that transmissions from users of the existing system are only occasionally received by the second system.
In order to increase transponder bandwidth utilization efficiency, many satellite communication systems operate in a demand assigned multiple access environment. Access request channels are used by a communications system to enable subscribers to be assigned satellite channel capacity for communications. For a random access request channel, such as slotted ALOHA, message loss increases rapidly when the number of offered messages is such as to approach full loading of the channel. As a result, nominal loading of a slotted ALOHA channel is usually held to about 20% of full loading.
An unslotted Aloha access request channel for similar reasons as for the slotted Aloha case, is designed to have an offered load of less than 0.1.
U.S. Pat. No. Re. 32,905 discloses a satellite communications system in which spread spectrum means are incorporated to enable a plurality of terminals, which feature low gain antennas, to concurrently generate spread spectrum CDMA transmissions over the same spectrum as used by existing systems, without interfering with users of the existing system. Furthermore, the system provides sufficient spread spectrum processing gain with its CDMA spread spectrum signal to substantially suppress interference caused by existing system transmissions. As a consequence of its processing gain, the system is able to process the CDMA spread spectrum transmissions generated concurrently by a plurality of terminals in the presence of interference from users of existing systems occupying the same spectrum, with an acceptable bit error rate, and without adversely affecting the users of the existing system. The techniques disclosed in the cited patent are applicable for those situations in which a sufficiently large spread spectrum processing gain can be achieved to substantially suppress existing system transmissions.
In the absence of sufficient spread spectrum processing gain to substantially remove interference caused by the transmissions of users of an existing system, the approach provided in the cited patent is ineffective. A processing gain that is insufficient to remove interference caused by users of existing systems may occur due a limitation in the bandwidth available for the spread spectrum signal, or by a relatively high information data rate, such that the ratio of spread spectrum bandwidth to information rate is too small to permit acceptable bit error rates in the presence of transmissions from an existing system.
An object of the invention is to alleviate this problem in a way which does not interfere with the primary users but still provides acceptable service to the secondary users.